1. Field of the Invention
This invention relates generally to a superconducting oscillator/counter analog-to-digital converter (ADC) and, more particularly, to a superconducting oscillator/counter ADC that simultaneously samples in-phase and quadrature-phase RF signals.
2. Discussion of the Related Art
There is a need for high performance and low power ADCs to digitize high frequency analog RF signals for high-speed signal processing applications, such as in radar and communications systems. ADCs of this type are particularly useful in receivers to sample and decipher a received RF signal. There is a push in the industry to move the digital conversion of the received RF signal farther up in the signal processing chain to provide greater processing capabilities. Superconductor ADCs have shown a great potential to achieve superior performance at much lower power than conventional integrated circuit semiconductor-based ADCs. Further, superconducting ADCs can operate at very high sample rates, for example, 10-100 GS/s, allowing conversion of very large bandwidth signals.
Superconductor ADCs employ superconducting logic circuits that use Josephson junctions instead of transistors as used in semiconductor-based circuits. A Josephson junction is a weak link between two superconducting materials where electrons tunnel across the junction. As long as the current through the junction is less than a critical current, the junction will be superconducting. A bias current is applied to the junction that is below the critical current. When additional current, for example, from an analog signal, is applied to the junction so that the current exceeds the critical current, the junction will generate a voltage pulse. The voltage pulse corresponds to a quantum leap in the magnetic phase of the junction, which will create a single flux quantum (SFQ) voltage pulse across the junction. The area of the SFQ voltage pulse generated at the junction is determined by fundamental physical constants and is h/2e, where h is Planks constant (6.6262xc3x9710xe2x88x9234 Joule seconds), and e is the fundamental electrical charge (1.602xc3x9710xe2x88x9219 Coulombs).
The SFQ pulses can be used to transmit data at very high frequencies. The SFQ pulses are transmitted by coupling a series of Josephson junctions together to provide a Josephson transmission line (JTL). When a particular Josephson junction in a JTL receives an SFQ pulse from a preceding Josephson junction, the pulse causes the junction to emit a voltage pulse, so that the SFQ pulse is recreated to continue propagating along the JTL. A discussion of JTLs operating in this manner can be found in U.S. Pat. No. 6,507,234, issued Jan. 14, 2003 to Johnson et al., assigned to the Assignee of this application, and herein incorporated by reference.
Oscillators/counter ADCs that use superconducting Josephson single flux quantum (SFQ) circuits for converting an analog signal to a digital signal are known in the art. U.S. Pat. No. 6,127,960 issued Oct. 3, 2000 to Silver et al. provides a discussion of superconducting oscillator/counter ADCs. FIG. 1 is a block diagram of a known superconducting oscillator/counter ADC 10 that is capable of simultaneously providing in-phase and quadrature-phase sampling of an RF analog input signal. The ADC 10 can be employed in any suitable superconducting circuit, such as a high frequency receiver that receives an RF analog signal to be digitized for further signal processing to extract the information thereon.
The RF analog signal to be digitized is applied to a superconducting voltage controlled oscillator (VCO) 12. The VCO 12 can be any superconducting circuit suitable for the purposes described herein. Superconducting VCOs of this type are known in the art, and may employ a single Josephson junction or multiple Josephson junctions for higher resolution. The VCO 12 generates a series of SFQ pulses, where the number of pulses generated for any given period of time is representative of the magnitude of the analog signal at that time.
The SFQ pulses from the VCO 12 are applied to a superconducting pulse splitter 14 that directs the SFQ pulses into an in-phase line 16 and a quadrature-phase line 18. The pulse splitter 14 is also a known superconducting circuit employing Josephson junctions that generates two SFQ pulses for each SFQ pulse it receives. The in-phase SFQ pulses on the line 16 are applied to an in-phase aperture gate 20 and the quadrature-phase SFQ pulses on the line 18 are applied to a quadrature-phase aperture gate 22. The aperture gates 20 and 22 are also superconducting logic circuits that employ Josephson junctions, and are well known to those skilled in the art. The aperture gates 20 and 22 are latch type devices that direct the input pulses to a positive or negative output line depending on their internal state. The aperture gates 20 and 22 toggle between their two internal states each time a clock pulse is received. For example, the aperture gates 20 and 22 can be single pole double throw (SPDT) SFQ pulse switches that toggle between two output lines.
The aperture gate 20 receives an in-phase SFQ clock signal from a gate control logic circuit 24 and the aperture gate 22 receives a quadrature-phase SFQ clock signal from the gate control logic circuit 24. The gate control logic circuit 24 is a superconducting logic circuit that generates the in-phase and quadrature-phase SFQ clock pulses based on SFQ clock pulses from a superconducting master clock 26. For each SFQ clock pulse from the master clock 26, the gate control logic circuit 24 alternately provides the in-phase SFQ clock pulses and the quadrature-phase SFQ clock pulses 90xc2x0 out of phase with each other.
FIG. 2 is a timing diagram showing the analog RF signal applied to the VCO 12, the SFQ master clock signal, the SFQ input pulses at the output of the VCO 12, the in-phase SFQ clock signal and the quadrature-phase SFQ clock signal. Each vertical line in the timing diagram represents an SFQ pulse. As is apparent, the spacing between the SFQ input pulses is determined by the magnitude of the analog signal, where the positive peaks of the analog signal provide closely spaced SFQ pulses and the troughs of the analog input signal provide farther spaced SFQ pulses.
The aperture gates 20 and 22 provide the SFQ input pulses on the positive and negative output lines. Each time an SFQ pulse in the in-phase clock signal and the quadrature-phase clock signal is received, the aperture gates 20 and 22 will cause the SFQ input pulses to be switched to the other of the positive output line or the negative output line. In other words, each time an SFQ clock pulse is received, the aperture gates 20 and 22 switch output lines.
The SFQ pulses on the positive and negative output lines from the aperture gate 20 are applied to a superconducting in-phase accumulator 32 and the SFQ pulses on the positive and negative output lines from the aperture gate 22 are applied to a superconducting quadrature-phase accumulator 34. When the SFQ pulses are provided on the positive line, the accumulator 32 or 34 increments or adds the pulses. When the SFQ pulses are provided on the negative line, the accumulator 32 or 34 decrements or subtracts the pulses. The positive regions (+) between the SFQ pulses in the in-phase clock signal and the quadrature-phase clock signal show the times that the SFQ pulses are being added or incremented, and the negative regions (xe2x88x92) in the in-phase clock signal and the quadrature-phase clock signal show the times that the SFQ pulses are being subtracted or decremented.
The gate control logic circuit 24 provides an in-phase integration clock signal to the accumulator 32 and a quadrature-phase integration clock signal to the accumulator 34. Each time an SFQ pulse in the in-phase integration clock signal and the quadrature-phase integration clock signal are received by the accumulators 32 and 34, respectively, the accumulators 32 and 34 output the accumulated SFQ input pulses therein as an in-phase digital sample and a quadrature-phase digital sample. The SFQ pulses in the in-phase integration clock signal and the quadrature-phase integration clock signal are sent to the accumulators 32 and 34 at the same time to provide the same sample period. The in-phase integration clock signal, the quadrature-phase integration clock signal, the in-phase digital sample signal and the quadrature-phase sample signal are also shown in the timing diagram in FIG. 2.
The direction of the arrowhead in the SFQ pulses in the in-phase sample line and the quadrature-phase sample line shows whether the SFQ input pulses are being added or subtracted in the accumulator 32 or 34. As shown, the timing of the in-phase clock pulses cause most of the input pulses to be added in the accumulator 32 relative to the peaks in the analog input signal, and the timing of the quadrature-phase clock signal provides an output of the accumulator 34 near zero.
Because the SFQ input pulses on the lines 16 and 18 are being continuously applied to the aperture gates 20 and 22, SFQ input pulses may arrive at the aperture gates 20 and 22 at the same time as SFQ pulses in the in-phase and quadrature-phase clock signals creating a violation in the timing conditions of the gate known as a pulse collision. Thus, when the aperture gates 20 and 22 are toggling between the positive and negative output lines, the arriving SFQ input pulse probably will not be counted by the accumulator 32 or 34. For example, if an SFQ input pulse on the line 16 arrives at the aperture gate 20 at the same time as an in-phase SFQ clock pulse arrives at the aperture gate 20, the SFQ input pulse may not be outputted on one of the positive or negative output lines to be accumulated by the accumulator 32. During these times, the behavior of the ADC 10 is undefined and errors may occur affecting the performance. Thus, the ADC 10 will not have a 100% duty cycle in that at those times when the aperture gates 20 and 22 are transitioning, the accumulator 32 and 34 will not be accumulating.
This problem is illustrated by the dotted oval in the timing diagram in FIG. 2. Particularly, an SFQ input pulse arrives at the aperture gate 22 at the same time that the aperture gate 22 is transitioning from the negative output line to the positive output line in response to a quadrature-phase SFQ clock pulse. This SFQ input pulse is not directed to either of the positive or negative output lines of the aperture gate 22, and thus does not have by an arrowhead. Therefore, this pulse is not accumulated by the accumulator 34.
In accordance with the teachings of the present invention, a superconducting oscillator/counter analog-to-digital converter (ADC) is disclosed that provides simultaneous in-phase and quadrature-phase sampling of an RF input signal. The RF signal is converted to a series of SFQ input pulses by a superconducting VCO. The SFQ input pulses are split by a superconducting splitter to be sent to an in-phase line that outputs the in-phase sample and a quadrature-phase line that outputs the quadrature-phase sample. The in-phase SFQ pulses are applied to an in-phase aperture gate and the quadrature-phase SFQ pulses are applied to a quadature-phase aperture gate. The in-phase aperture gate directs the in-phase SFQ pulses to a positive or negative output line in response to an in-phase SFQ clock signal. Likewise, the quadrature-phase aperture gate directs the quadrature-phase SFQ pulses to a positive or negative output line in response to a quadrature-phase SFQ clock signal. The positive in-phase SFQ input pulses are accumulated by an in-phase accumulator and the negative in-phase SFQ input pulses are decremented by the in-phase accumulator. Likewise, the positive quadrature-phase SFQ input pulses are accumulated by a quadrature-phase accumulator and the negative quadrature-phase SFQ input pulses are decremented by the quadrature-phase accumulator. In-phase SFQ sample clock pulses and quadrature-phase SFQ sample clock pulses are simultaneously applied to the in-phase accumulator and the quadrature-phase accumulator, respectively, to provide the in-phase digital sample and quadrature-phase digital sample.
The in-phase SFQ clock signal, the quadrature-phase SFQ clock signal, the in-phase sample SFQ clock signal and the quadrature-phase sample SFQ clock signal are generated by a superconducting gate control logic circuit. The gate control logic circuit receives an SFQ clock signal from a superconducting master clock circuit, and provides the in-phase clock signal and the quadrature-phase clock signal 90xc2x0 apart in phase. A pulse repulsion circuit is provided that receives the master SFQ clock signal and the SFQ input pulses. Within the repulsion circuit, the SFQ input pulses and the SFQ clock pulses repel each other so that they do not arrive at the in-phase aperture gate and the quadrature-phase aperture gate at the same time.
In one embodiment, the pulse repulsion circuit includes two Josephson junction transmission lines, where the SFQ input pulses propagate down one Josephson transmission line and the SFQ clock pulses propagate down the other Josephson transmission line. Magnetic coupling between inductors in the transmission lines causes the SFQ pulses to repel each other so that they reach the output of the transmission lines at different times. Therefore, the in-phase SFQ clock pulses will not arrive at the in-phase aperture gate at the same time as the in-phase SFQ input pulses. Likewise, the quadrature-phase SFQ clock pulses will not arrive at the quadrature-phase aperture gate at the same time as the quadrature-phase SFQ input pulses.
Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.